It is well known in the fiber industry that high modulus, high tenacity fibers can be prepared from rigid, wholly aromatic chain polymers such as aramid ("Kevlar", which is a registered trademark of E. I. duPont de Nemours & Co.). Although "Kevlar" has become widely used, its high cost limits its use primarily to specialty products.
Patents and scientific literature suggest that high strength fibers can be produced from flexible chain polymers, in particular polyethylene. As far as the current applicants are aware, however, no processes for producing flexible chain polymer fibers have been commercialized.
Processes for making fibers of polyethylene and other flexible chain polymers by solution spinning are well known. One example of such process is that described in U.S. Pat. No. 4,344,908 to Smith et al. According to Smith et al, a polyethylene gel fiber is formed by solution spinning, immediately cooled in a water bath at room temperature, and thereafter simultaneously heated, dried and stretched. A number of other references also describe solution spinning processes for producing polyethylene fibers in which heating, drying and stretching, or drawing as it is also known, are carried out simultaneously. For example, an article by Smith et al in Polymer, 1980, Vol. 21, November, pgs. 1341-1343, shows that solution spun polyethylene gel fibers which are simultaneously drawn and dried at elevated temperature immediately after they are generated have higher tensile strength at the same draw ratio than do filaments which are dried at room temperature and then subsequently drawn.
U.S. Pat. No. 4,413,110 describes processes for making high tenacity, high modulus polyethylene and polypropylene fibers by solution spinning, followed by cooling, solvent exchange and drying the fiber. The inventors report that they have a dense, void-free structure from the time of spinning until the dried, low-void fiber is obtained. This is achieved by slow cooling between the spinerette and the bath, which is achieved with a long air gap (at least 7.5 cm). In different modes of operation, the fiber is stretched either before solvent exchange, or after drying, or in two stages, one before solvent exchange and the other after drying. In each case the fiber is drawn in a heated tube. The methods of drying as reported in the examples are air drying at room temperature and vacuum evaporation at 22.degree.-50.degree. C. (No details as to drying are given except in the examples). Hot stretch (hot draw) feed roll speeds tend to be quite low, for example 2 or 4 cm/min.
European Patent Application, Publication No. 0055001 discloses a process for making filled polyethylene filaments by solution spinning followed by stretching. Stretching temperatures are from 75.degree. to 135.degree. C. The filament can be stretched without substantial solvent evaporation, or a portion of the solvent may be removed (preferably to less than 10 percent by weight of solvent) by various means such as washing, evaporation or hot air drying, followed by stretching. Filament speeds are not disclosed.
None of the drying conditions employed in the references cited above lend themselves to high speed continuous processing.